A hallmark of the vertebrate central nervous system is its extreme complexity. Complexity is exhibited at the level of synaptic connections as well as in the number of cell types. The developmental mechanisms that generate such diversity are just beginning to be explored. Lineage analysis has been a valuable technique in addressing this problem. Studies in the retina and cortex suggest cell fate is determined at least partially by local environmental cues operating near the time when a cell is born. In contrast, lineage analysis in the brain stem suggests that classical developmental compartments, in which the clonally-related progeny are restricted to one rhombomere, may exist. The thalamus, which plays a central role in integrating the hindbrain and forebrain, appears to have developmental and anatomic features which overlap with both the hindbrain and the forebrain. To date, no studies have explored lineal relationships in the thalamus. Given the anatomic similarities to both regions, lineage analysis in the thalamus potentially could provide valuable insights into the strategies determining cell fate in the central nervous system. To determine lineal relationships in the thalamus, embryonic chick brains will be infected with a library of replication incompetent retroviruses modified by inserting the alkaline phosphatase gene or the E. coli lacZ gene and a unique molecular tag. Each brain will be serially sectioned and cells infected by the vector identified by using enzyme histochemistry for the transduced alkaline phosphatase or beta-galactosidase. Positive cells will be 'picked' from the slide and amplification of the unique molecular tag will be accomplished using the polymerase chain reaction (PCR). A statistical analysis will then be conducted to determine whether all cells with identical tags are siblings. This technique will establish the location of cell proliferation, migrational pathways, physical boundaries, and cell type within clones in the thalamus. Finally, we hope to investigate some of the environmental and genetic signals which determine cell fate in the thalamus. Results from this study could eventually lead to a better understanding of development in all parts of the CNS and insight into why certain nuclear groups and cell classes are affected in different diseases.